The present invention has particular application to motorcycles commonly referred to as "dirt bikes" or mountain bike which are typically ridden over rough terrain and/or on steep uphill or downhill slopes.
Of particular concern to the "off-road" motorcycle rider is the jolting of the wheels as disturbances in the ground surface, such as rocks, holes, or vertical drops, are encountered. This jolting is transferred to the rider through the wheels, fork, frame, and ultimately the rider's body. In addition to the potential discomfort to the rider, there is the added concern for safety. That is, the motorcycle becomes difficult to control at the instant of a severe jolt being experienced by the rider through the handles, and frame.
To overcome this problem of severe jolting, designers of motorcycles have developed front and rear wheel, shock absorbing suspension systems. These shock absorbing systems include a rigid rod (or inner tube), which is slidable within a rigid sleeve (or outer tube) and a biasing member which can operate pneumatically, hydraulically, elastomerically or with metallic springs, positioned within the rigid sleeve to achieve the "shock-absorbing" action. The biasing member extends the fork rods relative to the sleeves, and as obstacles are encountered by the front or rear wheel, the biasing members of the fork's rigid sleeves collapse as the slidable rods are compressed in the sleeves, thereby absorbing the severe jolt. Additionally, the sliding rod may have a slight degree of rotatability within the sleeve.
Therefore, when an obstacle is encountered directly, the forces are substantially in the same axis as the suspension system, and the slidable rods are typically displaced uniformly. However, during cornering or other maneuvering, the forces are not in the same axis as the suspension such that torsional and lateral stresses are created, and typically one of the rods is compressed or displaced into the corresponding sleeve more so than the opposite rod. Because the slidable rod has a slight degree of rotatability within the sleeve, high stresses are created at the dropout-to-axle connection when lateral and torsional forces are applied to the wheel in contact with the ground, such as in cornering.
Also, when brakes are applied in stopping or cornering, the brakes push outward and a large amount of torsion acts on the lower two fork tubes. The resistance to this torsion is mainly provided by the wheel axle and the brake arch. Thus, increased rigidity and strength are highly desired throughout the front fork, especially at the wheel axle, brake arch and/or crown regions.
One new design to increase the strength and rigidity at the wheel axle comprises an enlarged wheel hub and axle (the axle being approximately 20 mm in diameter). While uniform compressions relieve stress on the wheel, fork, steering tube, and steering handle, unbalanced compressions, such as from cornering and maneuvering, the stresses on the wheel axle remain high. Therefore, increased rigidity and strength in the wheel axle is highly desirable for off-road motorcycles. Such a novel design in discussed in co-pending application Ser. No. 09/189,448, which is herein incorporated by reference. However, there remains a need to provide a suspension system which will reduce compression forces incurred during ordinary use of the motorcycle.
Furthermore, an additional concern for motorcycle riders is the weight of the bicycle or motorcycle. A lighter motorcycle is more desirable because it takes less force to power and maneuver. As such, manufacturers of high-end performance bikes and their components are continuously upgrading them to decrease the overall weight. This has typically been accomplished in at least three ways. One is to use lighter materials such as aluminum alloys and carbon-graphite components. Another is to decrease the overall number of components that comprise the bicycle or motorcycle. Yet another is to decrease the thickness of the components used without sacrificing their strength.
An example of such an improvement is described in co-pending application Ser. No. 09/236,998 (the "'998 application"), which is herein incorporated by reference. In this case, separate components are combined at the front fork suspension system of the bicycle. Typically, a front fork suspension system includes a crown which allows attachment of the central steering tube and a pair of parallel fork legs which each comprise an outer rigid sleeve and inner rigid rod which are slidably engaged with each other. Conventionally, the crown is connected to the fork legs via an adhesive or some form of mechanical connection (i.e., screws). In one conventional system, the crown is provided with three openings wherein the outer two openings has slits near each of the outer ends. The center opening receives the steerer tube, usually press fit into this opening. The outer two opening receive the upper ends of the parallel fork legs, usually the upper ends of the inner rigid rods. Then, screw(s) or some other threaded fastening device(s) is employed to close or clamp the crown openings onto the upper ends of the inner rigid rods to form a tightly secured attachment.
Although quite simple, this mechanical means for attaching the fork legs to the crown has several disadvantages, some of which include increased overall weight of the forks, increased number of stress points as well as increased stresses at these points, decreased overall strength and stability, etc. The invention of the '998 application has overcome these shortcomings through a novel method of manufacturing the crown and legs as a single component. This "monolithic" crown/fork design results in an overall lighter fork due to the elimination of extra components, increased strength and stability due to the elimination of high stress locations as well as a reduction in the stresses created at the interface of multiple components.
Also, in conventional suspension forks, a brake arch is typically mounted on the upper portion of the lower fork legs via screws or other threaded fastening devices, one on each rod, while the brake arch has receptacles for mounting brake calipers. In an alternative design, known as an upside down (or inverted) fork, the outer rigid sleeves and inner rigid rods are reversed. More specifically, the crown is connected to the outer rigid sleeves rather than to the inner rigid rods as previously described. This inverted design provides greater strength and stability at the crown/fork leg interface.
Additionally, recent trends show an increased use of disc brakes on motorcycles due to their increased performance and high durability. Typical disc brake systems are mounted on one of the dropouts at the wheel axle for maximum performance. Conventionally, the disc brakes comprise a separately mounted caliper containing the brake pads which, when in the closed position (i.e., pressed together), provide a high degree of frictional force to slow the wheel to a stop. It is therefore appreciated that the present invention can be used with motorcycles having either a conventional brake arch design or a conventional disc brake system.